underrunner

plural underrunners

1. The lower of a pair of grinding millstones.

   The arrangement is preferably applied to an underrunner mill.

   1896, Great Britain. Patent Office, Patents for Inventions. Abridgments of Specifications, page 11

   1 more example

   FOR SALE AT A BARGAIN. […] Four run stones, Munson underrunners, 3 ft diameter.

   1890, The Northwestern Miller, volume 29, page 563

2. An additional runner added to the bottom of a sledge to protect the main runners in harsh conditions.

   Less than half a mile to the south the party sighted an underrunner of a sledge, and assumed it to mark the location the Norwegians had assigned for 90°S.

   2002, Paul Simpson-Housley, Antarctica: Exploration, Perception and Metaphor, page 36

3. A runner (mechanical part intended to guide or aid something else to move) designed to be inserted underneath the item to be moved.

   A triple tandem tow using underrunners. This method has been used for long ocean tows by tugs with a single-drum winch.

   1989, John S. Blank, Modern Towing, page 302